The present invention relates in general to a gate driver for the devices connected to the non-reference DC rail in a power converter and more specifically to driving devices exhibiting a high impedance control input without a floating power source.
Power converters are known in the art for a wide variety of uses, including power supplies for arc lamps. In a half-bridge converter, a single leg comprising a series-connected pair of switching devices is connected across a DC supply voltage, the output of the leg being the junction between the switching devices. In a full bridge converter, two legs are connected in parallel, the output of the converter being taken between the outputs of the two legs to supply an AC voltage. Multiphase AC voltages may be provided by connecting additional legs.
A problem associated with the design of DC to AC converters relates to the switching of the upper devices, i.e. the devices connected to the non-reference DC rail (usually the positive rail), which are not connected to circuit common. In conventional converters with bipolar transistors used as the switching devices, the drive voltage for the upper devices must exceed the main DC supply voltage since a continuous current must be supplied to the base of each device while it is turned on. In other words, since the voltage between the base and the negative electrode of a bipolar transistor is typically much greater than the voltage between the positive and negative electrodes when the transistor is on, the drive voltage between the transistor gate and circuit common, for an upper device, must be greater than the DC supply voltage. The excess voltage is typically supplied by a separate, floating power supply. The special level shifting circuitry required to drive the upper devices complicates the converter, with a consequent increase in the cost of the circuit.
Other switching devices exhibiting a high impedance control input, such as field-effect transistors (FETs) and insulated-gate transistors (IGTs), have also been used in converters. These devices can be turned on by application of a voltage signal to the control input, rather than a current signal as required by bipolar transistors. U.S. Pat. No. 4,485,434, issued to Beeston et al. on Nov. 27, 1984, shows a full bridge converter using FETs and supplying an arc lamp, but also requires a transformer with four separate secondary windings and four separate voltage regulators for driving the FETs.
Accordingly, it is a principal object of the present invention to provide a driver circuit for the upper device in a converter leg which eliminates the need for a separate floating power source for the driver circuit.
It is another object of the present invention to provide a DC to AC converter specially adapted to operate an arc discharge lamp.